For example, from the viewpoint of power conversion efficiency, a transmission unit such as a base station in a mobile communication system may be designed to operate in a nonlinear region where back-off from saturation power is small. Thus, when a modulating signal for high-speed wireless communication is inputted, out-of-band radiation due to nonlinear distortion occurs, thereby affecting adjacent transmission channels.
Distortion compensation technology for reducing such out-of-band radiation caused by a transmission amplifier includes predistortion (PD) compensation in which the reverse characteristic of the transmission amplifier is added in advance to transmission signals. In the predistortion compensation, a distortion compensation coefficient is adaptively updated so as to reduce the difference between a transmission signal and a signal obtained by feeding back part the output signal of the transmission amplifier. In addition, digital predistortion is known in which predistortion is performed in a digital region.
A transmission amplifier such as a base station may be implemented by multi-stage connection of a plurality of amplifiers. When the transmission amplifier having such a multi-stage structure is operated with high power conversion efficiency, nonlinear distortion occurs also in an amplifier other than the final stage amplifier, and thus a signal distorted in a former stage amplifier (for example, a driver stage amplifier) is inputted to the final stage amplifier, thereby causing complicated nonlinear distortion in the final stage amplifier. For this problem, a distortion compensation circuit is known in which a predistortion compensation unit for compensating the former stage amplifier, and a predistortion compensation unit for compensating the final stage amplifier are connected in cascade (for example, see Japanese Laid-Open Patent Publication No. 2006-279633).